FIG. 4 shows a circuit block diagram of a conventional photoelectric conversion integrated circuit device that employs a photodiode as a light-sensing device. In this figure, reference numeral 1' represents a photodiode that outputs a current signal in accordance with a light signal it receives, reference numeral 2 represents a current-to-voltage conversion circuit (hereafter referred to as the "I/V conversion circuit") that outputs a voltage in accordance with a current signal it receives, and reference numeral 3 represents a test circuit composed of a switching device 31, a constant-current circuit 32, and a switch driving circuit 33.
To the input of the I/V conversion circuit 2, the output (cathode) of the photodiode 1' is connected, and the constant-current circuit 32 provided within the test circuit 3 is also connected thereto through the switching device 31. Thus, when the photodiode 1' senses a light signal L, or when the switching device 31 is turned on, a current flows from the I/V conversion circuit 2 to the photodiode 1' or to the switching device 31, and a voltage corresponding to this current is fed out via a terminal T.sub.o. Within the I/V conversion circuit 2, as shown in FIG. 5, a reference voltage Vref is applied to the non-inverting input terminal (+) of an operational amplifier via a terminal 21 through a resistor R1, and the cathode of the photodiode 1' is connected via a terminal 22 to the inverting input terminal (-) the operational amplifier. Between this inverting input terminal (-) and the output terminal T.sub.o, a resistor R2 is connected. When an output current flows through the photodiode 1', the current I flows through the resistor R2, and thus a voltage I.times.R2 appears at the output terminal T.sub.o.
In FIG. 4, the switch driving circuit 33 controls the on/off state of the switching device 31; specifically, when the switch driving circuit 33 receives a predetermined voltage at its test pin T.sub.T, it turns on the switching device 31.
In general, an integrated circuit device is subjected to a functioning check in its manufacturing process. In a functioning check of a photoelectric conversion integrated circuit device, it is desirable to shine light on the photodiode (light-sensing device) 1'; however, in reality, it is difficult to shine a predetermined amount of light on the photodiode 1'. For this reason, a functioning check is achieved by externally turning on the switching device 31 provided within the test circuit 3 so that the constant-current circuit 32 will cause the I/V conversion circuit 2 to output a predetermined amount of current as much as it outputs when a predetermined amount of light is shone on the photodiode 1'.
Here, if there is a break at point A' shown in FIG. 4 in the wiring between the photodiode 1' and the I/V conversion circuit 2, even if the photodiode 1' senses light, the I/V conversion circuit 2 outputs no current I; that is, the photoelectric conversion integrated circuit device is defective.
However, in the functioning check described above, unless there is a fault in the wiring between the I/V conversion circuit 2 and the test circuit 3, the I/V conversion circuit 2 outputs a current, and therefore, unless any fault is found in other respects such as the characteristics of the I/V conversion circuit 2, the photoelectric conversion integrated circuit device passes as acceptable.
In this way, with a conventional photoelectric conversion integrated circuit device, even if a functioning check is conducted in the manner described above, i.e. by feeding an input current to the I/V conversion circuit without shining light on the light-sensing device, it is impossible to detect a fault in the wiring between the photodiode 1' and the I/V conversion circuit 2.